MORPHOLOGY OF HATSCHEK NEPHRIDIUM IN LARVAL AND JUVENILE STAGES OF BRANCHIOSTOMA-VIRGINIAE (CEPHALOCHORDATA)

The unpaired Hatschek's nephridium is the largest and most anterior of the nephridia in the body of Branchiostoma virginiae. Its single coll ecting tubule lies on the left side of the notochord along the left an terior aorta. The nephridium extends from the anterior vestibule to a point immediately posterior to the velum where it opens into the endod ermal pharynx. Along its length, the collecting tubule gives off clust ers of filtration cells (cyrtopodocytes) that radiate from the tubule and enter small urinary capsules of coelomic origin. The capsules lie directly on the wall of the aorta. Filtration-cell perikarya and mesot helial cells comprise the epithelial lining of the capsules. The basal surface of each filtration cell produces podocyte-like pedicels over the wall of the aorta and adjacent connective tissue. Apically, each f iltration cell resembles a solenocyte. The apical surface bears a flag ellum enclosed in a collar of 10 long microvilli, which are bridged by extracellular matrix. The flagellated collars traverse the capsule an d insert into the wall of the collecting tubule. Presumably, blood and interstitial fluid are ultrafiltered into the capsules by the capsula r basal lamina; flagellar beat may augment ultrafiltration. Filtration cells, and especially cells of the collecting tubule, are absorptive epithelial cells. Hatschek's nephridium in 3-gill-slit-stage larvae li es dorsolateral to the mouth, between myotomes 1 and 2, along the left anterior aorta. The nephridiopore opens through endoderm into the pha rynx inside the dorsal lip. The short collecting tubule produces a clu ster of filtration cells that protrude into and join the epithelial li ning of the myocoel of myotome 1 (= somite 2, left side). The filtrati on cells produce pedicels over the surface of the left anterior aorta; an apical collared flagellum penetrates and enters the collecting tub ule.Hatschek's nephridium is a serial homolog of the paired, posterior , branchial nephridia. The similarity in spatial relationship of Hatsc hek's nephridium with the mouth/velum and of a branchial nephridium wi th its gill cleft suggests that the mouth/velum is a specialized gill cleft. Hatschek's nephridium, like the paired branchial nephridia, is intermediate in structure between a protonephridium and a metanephridi al system suggesting functional and phylogenetic implications for this design.